Pressure sensor system

ABSTRACT

A pressure sensor that responds to a predetermined rate of change of pressure in the fluid medium comprises a housing that encloses a pressure chamber which has a permeable wall and flexible diaphragm that separates the pressure chamber from the pressure sensitive system in which the sensor is used. The sensor may be used in a pneumatic tire and in such use the flexible diaphragm will be deflected to activate a transmitter when the pressure in the tire changes at a hazardous rate. The sensor will not activate the transmitter when the rate of change of pressure is less than a hazardous rate of change.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 07/447,807 filed Dec. 8, 1989, now U.S. Pat. No. 4,975,679,which is in turn a continuation in part of U.S. patent application Ser.No. 07/202,262 filed June 6, 1988, now U.S. Pat. No. 4,890,090.

BACKGROUND OF THE INVENTION

This invention relates to a pressure sensor that responds to apredetermined rate of change of pressure in a fluid medium of a pressuresensitive system. In particular, this invention relates to a pressuresensor which may be used in a tire pressure warning system forgenerating an alarm signal when the rate of change of pressure in thetire exceeds a hazardous rate of change.

DESCRIPTION OF RELATED ART

Pressure sensitive switches that respond to pressure changes in order togenerate an alarm are well-known. The present pressure sensor, however,responds to a predetermined rate of change of pressure in a fluid mediumrather than a simple pressure differential.

U.S. Pat. No. 4,211,901 Matsuda is directed to a pressure sensing switchfor transducing the change of pneumatic pressure to an electricalsignal. This device responds to the pressure differential between twochambers that are separated by a diaphragm. The device is designed torespond to the pressure differential between the two chambers and makesno allowance for circumstances where the rate of change of pressure inthe pressure sensitive system which is being monitored is such that itdoes not represent the hazard.

U.S. Pat. No. 4,048,614 Shumway discloses the pressure detector andradio transmitter system for providing a tire pressure warning. Thissystem is designed to generate an alarm when the air pressure dropsbelow a set minimum operating pressure.

While it is important under certain circumstances to generate an alarmwhen the pressure in the pneumatic tire drops below a predeterminedcritical pressure, there are circumstances under which the merereduction in pressure in the tire below a predetermined level is nothazardous.

The wall of a pneumatic tire and the seal formed between the tire andthe rim are permeable to a certain extent and as a result the pressurein an inflated tire will fall over an extended period of time. This verygradual reduction in pressure is not hazardous when, for example, theinflated tire is mounted on a trailer of a vehicle which is held instorage for a matter of several months. It is quite common to havetrailers stored in a marshalling yard for extended periods of time andif the tires of these vehicles were fitted with tire pressure alarmsthat responded to a reduction in the pressure in the tire, these alarmswould go off frequently. Because these alarms are likely to be poweredby a low-powered battery, the batteries are likely to run down when thealarms go off and are unattended with the result that it would benecessary to replace the batteries before the device could beeffectively reactivated. This involves removing the tire from the rim.In some circumstances the fact that the alarm has gone off and thebattery has discharged may not be readily evident and this could givethe operator a false sense of security in that the operator is not awareof the fact that the device is inoperable.

In most motor vehicles a slow loss in the pressure in a tire is nothazardous. As previously indicated, all tires are permeable to someextent with the result that air is escaping on a continuous basis fromvirtually all inflated pneumatic tires. This loss in pressure onlybecomes hazardous when the rate at which it occurs is such that thepressure in the tire can drop below a critical pressure during on-roaduse.

SUMMARY OF INVENTION

It is an object of the present invention to provide a pressure sensorthat responds to a predetermined rate of change of pressure in a fluidmedium of a pressure sensitive system.

It is a further object of the present invention to provide a pressuresensor in which a pressure chamber is formed and wherein one of thewalls of the chamber is a permeable wall which serves to permit thepressure in the pressure chamber to change at a predetermined rate andwherein one of the walls of the pressure chamber is a flexible diaphragmthat moves to a predetermined position in response to a predeterminedchange in the pressure differential between the pressure chamber and thepressure sensitive system with which the sensor communicates.

It is a further object of the present invention to provide a tirepressure alarm system that incorporates a pressure sensor that respondsto a predetermined rate of change of the air pressure in the tire.

According to the present invention, there is provided a pressure sensorswitch comprising a stem having a side wall surrounding a vent chamberthat opens into one end of the stem, a vent passage opening from thevent chamber through the side wall of the stem, a tubular sleeve mountedon such stem and overlying said vent passage, said tubular sleeve beingflexible so as to be movable to and fro between a first position inwhich it extends in close proximity to said vent passage and a secondposition which is spaced outwardly from the the first position withrespect to the stem in response to a variation in pressure in the ventchamber, said sleeve being electrically conductive and forming a firstterminal of said switch, a second terminal of said switch being locatedopposite said tubular sleeve so as to be spaced therefrom when thesleeve is in the said first position and to contact the sleeve when thesleeve is in said second position to open and close said switch inresponse to movement of said sleeve.

PREFERRED EMBODIMENT

The invention will be more clearly understood after reference to thefollowing detailed specification read in conjunction with the drawingswherein;

FIG. 1 is a sectional side view of a pressure sensitive electricalswitch constructed in accordance with an embodiment of the presentinvention,

FIG. 2 is a diagram illustrating a pressure sensitive transmitterassembly incorporating the pressure sensitive switch of FIG. 1,

FIG. 3 is a diagrammatic representation of a receiver for use inassociation with the transmitter of FIG. 1,

FIG. 4 is a sectional view taken through an inflated wheel assemblyillustrating the manner in which the transmitter and pressure sensitiveswitch are mounted within the inflation chamber of a tire,

FIG. 5 is a pictorial view of the mounting bracket of FIG. 4,

FIG. 6 is a partially sectioned pictorial view of a pressure sensitivetransmitter assembly,

FIG. 7 is a sectional side view through a portion of a pressuresensitive switch constructed in accordance with a further embodiment ofthe present invention,

FIG. 8 is a sectional view taken through a membrane suitable for use inassociation with the switch constructed in accordance with FIG. 7.

FIG. 9 is a sectional side view of a tire pressure arm systemconstructed in accordance with a further embodiment of the presentinvention.

FIG. 10 is a diagrammatic representation of a pressure sensorconstructed in accordance with a still further aspect of the presentinvention.

FIG. 11 is a diagrammatic representation of a pressure sensorillustrating a still further embodiment of the present invention.

FIG. 12 is a diagrammatic representation of a pressure sensorillustrating a still further aspect of the present invention.

FIG. 13 is a diagrammatic representation of a pressure sensorillustrating yet another aspect of the present invention.

FIG. 14 is a circuit diagram illustrating a temperature switch arrangedin series with the transmitter.

FIG. 15 is a sectional side view of a pressure sensor constructed inaccordance with a further embodiment of the present invention.

FIG. 16 is a pictorial view of the top surface of a divider wallsuitable for use in a pressure sensor of the type illustrated in FIG.15.

FIG. 17 is a plan view of the bottom face of the divider wall of FIG.16.

FIG. 18 is a sectional side view of a pressure relief valve constructedin accordance with a further embodiment of the present invention.

FIG. 19 is a sectional side view of a pressure sensor constructed inaccordance with yet another embodiment of the present invention.

FIG. 20 is a plan view of the P.C. Board of FIG. 19.

FIG. 21 is a sectional side view of a pressure sensor constructed inaccordance with a still further embodiment of the present invention.

With reference to FIG. 1 of the drawings, the reference numeral 10refers generally to a pressure sensitive electrical switch constructedin accordance with an embodiment of the present invention.

The pressure sensitive electrical switch 10 comprises a housing 12 whichis constructed from three components which are identified by thereference numerals 14, 15 and 16. The component 14 is acylindrical-shaped sleeve member which has a bore 20. The component 14is made from a material which will act as a conductor such as brass orcopper.

The component 16 is formed from an insulating material and has acylindrical side wall 26 and an end wall 28. The cylindrical side wall26 has an end face 30 which is outwardly and rearwardly inclined towardthe end wall 28. The cylindrical side wall 26 is proportioned to fit inan interference fitting relationship within the bore 20 of the component14. The component 15 is formed from an insulating material and has acylindrical side wall 24 and an end wall 18. The side wall 24 has an endface 22 which is outwardly and rearwardly inclined toward the end wall18. A flexible diaphragm 32 is formed with a perimeter mounting ring 34and a thin flexible membrane 36. The mounting ring 34 is wedged betweenthe end face 22 and the end face 30. It will be seen that the mountingring 34 will be urged into contact with the inner face of the component14 to form a good electrical contact therebetween.

The components 14, 15 and 16 and the flexible diaphragm 32 are assembledas shown in FIG. 1 such that the membrane 32 serves to separate thefirst pressure chamber 38 from the second pressure chamber 40. Thesecond pressure chamber 40 is a sealed chamber. The first pressurechamber 38 has an access passage 42 which extends through a nipple 44which is initially closed by a frangible end wall 46. The components 14,15 and 16 and the diaphragm are assembled in an environment which ispressurized to the ambient or control pressure which is required in thesecond chamber 40 in use. When the device is to be used in a tire of atruck or the like the control pressure is about 100 psi.

A first electrical conductor 48 is mounted in the end wall 28 andprojects into the first pressure chamber 38. The inner end 50 of thefirst electrical conductor 48 is arranged to spaced from the diaphragm32 and serves to form a first electrical contact.

A second electrical conductor 56 is mounted in the end wall 18 andprojects therethrough. The second electrical conductor 56 serves toprovide a second contact 58 which is arranged in a spaced relationshipwith respect to the membrane 36 when it is in its normal positionillustrated in FIG. 1. Preferably, the contacts 50 and 58 are equallyspaced from the membrane 36. It will be apparent that when a pressuredifferential is established between the chambers 38 and 40, the flexiblemembrane 36 will be deflected toward the low pressure chamber and when apredetermined pressure differential is established, the membrane 36 willmake contact with one or other of the contacts 50 or 58 to effectivelyclose the switch to permit current to flow through the component 14,membrane 36 and either of the conductors 48 or 56 depending upon whichof the conductors is in contact with the diaphragm 36.

The flexible diaphragm 32 is made from an electrically conductivesilicone rubber material such as that manufactured by Armet IndustriesCorporation of Tilsonburg, Ontario, Canada. This material is normallyconsidered to be substantially impermeable to air, however, a signficantamount of air may permeate the membrane 36 if a pressure differential ismaintained between the chambers 38 and 40 for an extended period of time(i.e. several days). Because of the gradual equalization of pressureresulting from the permeability of the membrane 36 the membrane will notbe deflected to contact the conductors unless the rate of charge of thepressure in the pressure chamber 40 exceeds the rate at which the fluidmedium permeates the membrane 36.

By pressurizing the chambers 38 and 40 to an initial pressure which isclose to the operating pressure in the system which is to be monitored,the membrane 36 will assume the neutral position. When the switch is tobe used, the end wall 46 is removed from the nipple 44 to open thepassageway 42. The passageway 42 is then placed in communication withthe pressure system which is to be monitored so that the pressure in thesystem is applied to the first pressure chamber 38. If the pressuredifferential between the chambers 38 and 40 is greater than that withinwhich the switch is designed to remain open, the flexible membrane 36will be deflected into contact with one or other of the contacts 50 or58 as previously indicated to close the switch 10.

When the pressure sensitive switch 10 is assembled in a pressurizedenvironment, the chambers 38 and 40 are pressurized to a pressure whichis substantially equal to the normal operating pressure. Any pressuredifferential which is initially present will gradually diminish as aresult of the fact that the air will eventually permeate the membrane36. The air pressure is maintained in the chambers 38 and 40 by reasonof the end wall 46 which closes the passage 42.

As shown in FIG. 2 of the drawings, the switch 10 may be used toadvantage in a pressure sensitive transmitter assembly 60. Thetransmitter assembly 60 includes a power source in the form of anelectrical battery 62 and a transmitter 64. A pressure sensitivetransmitter assembly 60 of the type described in FIG. 2 of the drawingswhich is to be used in a tire pressure warning system may employ abattery such as one or more double-A 1.5 volt long-life batteries and atransmitter in the form of a commercially available short-wave radiotransmitter such as the type commonly used to control the opening andclosing of garage door mechanisms or the like.

A portable receiver suitable for use in association with the transmitterassembly of FIG. 2 is generally identified by the reference numeral 70in FIG. 3 of the drawings. The receiver 70 may be of any conventionalconstruction compatible with the transmitter 42. The receiver 70 has anaudible alarm 72 and a visual alarm 74. The audible alarm 72 may be inthe form of an alarm buzzer and the visual alarm may be in the form ofan LED. A cancellation button 76 is provided for interrupting the alarmcircuit of the receiver. Receivers which generate audible alarms andactivate visually detectable indicators in response to the receipt of apredetermined signal are well known and will not therefore be describedin detail. The receiver of the present invention is mounted in aportable housing 78 in which a conventional pressure gage 80 may also bemounted. The pressure gage 80 has a connecting nipple 82 of a typesuitable for forming a connection with the inflation valve of a tire orthe like. The pressure gage 80 is designed to give a reading of thepressure detected when the connecting conduit 82 is coupled to a valveof a tire or the like.

As shown in FIG. 4, the pressure sensitive transmitter assembly 60 ismounted within the inflation chamber 84 of a pneumatic tire assembly ofthe type generally identified by the reference numeral 86. In thisembodiment the transmitter assembly 60 is mounted on a bracket 88 whichis in turn mounted on the inner end 90 of the valve stem 92 which ismounted on the well rim 94.

The mounting bracket 88 is illustrated in FIG. 5 of the drawings whereinit will be seen that it is shaped to provide a seat 96 and a mountingflange 98 in which a mounting passage 100 is formed. The mountingpassage 100 is proportioned to receive the valve stem 92. The seat 96has a generally dove-tail configuration.

With reference to FIG. 6 of the drawings, it will be seen that thetransmitter assembly 60 is mounted in a housing 102 which has a baseportion 104 which has a dove-tail cross-sectional configurationcorresponding to that of the seat 88. The battery 62, transmitter 64 andpressure-sensitive switch 10 are all accommodated within the housing 102with the nipple 44 projecting through a wall of the housing 102 so thatthe through passage 42 will open into the inflation chamber 84 (FIG. 4)when the end wall 46 (FIG. 1) is removed.

A pressure sensitive switch constructed in accordance with a furtherembodiment of the present invention is illustrated in FIG. 7 of thedrawings to which reference is now made. In the embodiment illustratedin FIG. 7 of the drawing, conductors 110 and 112 are mounted in the endwall 114 in a spaced parallel relationship to provide contacts 116 and118. Similarly, conductors 120 and 122 are mounted in the side wall 124to provide terminals 126 and 128 on the opposite side of the diaphragm132. An input passage 142 extends through a nipple 144 into the firstchamber 138. The through passage 142 is normally closed by an end wall146. In this embodiment, the diaphragm 132 is deflected into contactwith the contacts 116, 118 or the contacts 126, 128 depending upon thedirection in which the diaphragm is deflected. When the diaphragm makescontact with the contacts 116, 118, current can flow from the conductor110 to the conductor 112. Similarly, when the diaphragm 132 makescontact with the contacts 126 and 128, current can flow through theconductor 120 to the conductor 122.

In a still further modification which is not shown, only one set ofcontacts 116, 118 may be provided on one side of a diaphragm so thatmovement of the diaphragm can make and break the contact between thecontacts 116 and 118 as required in use.

A diaphragm suitable for use in the embodiments illustrated in FIG. 7 ofthe drawings is shown in cross-section in FIG. 8. The diaphragm includesan annular perimeter ring 150, a flexible membrane 152 and a centralboss 154. As previously indicated, the diaphragm is made from a flexiblematerial which is capable of conducting electricity. When the annularring 152 is clamped between the shoulder 24 and end face 30 (FIG. 1), itwill be deformed to assume the generally triangular configurationillustrated in FIG. 1 and this will serve to lock the ring 150 in thehousing.

The boss 154 is particularly suitable for use in the embodimentsillustrated in FIG. 7 of the drawings wherein it is desirable to provideflat end faces 156 and 158 in order to establish contact between thecontacts 116, 118 and between the contacts 126, 128 as previouslydescribed. The boss 154 and the membrane 152 are each circular in frontview.

As previously indicated, when the pressure sensitive switch is initiallyassembled, the chambers 38 and 40 are pressurized to a pressure which isabout equal to the pressure in the environment in which the pressuresensitive switch is designed to operate in use. In a typicalinstallation such as the inflation chamber of a pneumatic tire such asthat used in the trucking industry, the chambers 38 and 40 would bepressurized to about 100 psi gage. The pressure sensitive switch can bestored in this condition for an extended period of time because littleor no load will be applied to the membrane because there is little or nopressure differential between the pressure chambers 38 and 40 and anypressure differential which may initially exist will diminish by reasonof the permeability of the membrane 36. When the pressure sensitiveswitch is to be used in the manner illustrated in FIG. 4 of thedrawings, the end wall 46 of the nipple is removed to open the passage42. As a result, the pressure in the chamber 38 will drop and themembrane 36 will be deflected to make contact with the contact 50. Thisprocedure can be used to test the transmitter to ensure that it willfunction to emit a warning signal. When the pressure sensitive switch isinstalled, the tire is then inflated and inflation will continue untilthe required pressure is established within the inflation chamber 84.This pressure will be transmitted to the pressure chamber 38 of thepressure sensitive switch and will serve to deflect the membrane 36 backto the neutral position. If the tire is over-inflated, the membrane 36will be deflected to make contact with the contact 58 and this willagain activate the transmitter to generate a warning signal. After thetire has been correctly inflated, the membrane 36 will be located in aposition which is very close to the neutral position shown in FIG. 1 ofthe drawings. If there is a pressure differential between the pressurein the inflation chamber 84 and that in the reference chamber 40, thepressure differential will be reduced in time by reason of thepermeability of the membrane 36. As previously indicated, even membraneswhich are considered to be substantially impermeable are not totallyimpermeable with the result that some transfer of air will take placeover an extended period of time resulting in the elimination of pressuredifferentials which are maintained over a long period of time. If,however, the pressure in the inflation chamber 84 decreases or increasesto an undesirable extent at a rate which is greater than the rate atwhich air can permeate the membrane 36, the membrane 36 will bedeflected to form contact with the contact 50 or 58 to activate thetransmitter which will in turn emit a warning signal which will activatethe alarm mechanisms of the receiver as previously described.

A further embodiment to the present invention is illustrated in FIG. 9on the drawings wherein reference number 160 refers generally to apressure sensor and alarm system for use in a pneumatic tire.

The pressure sensor system 160 includes a housing 162 in which apressure chamber 164 is formed. A pressure sensitive electrical switchwhich is generally indentified with reference number 166 is mounted atone end of the housing 162 and serves to form a wall of the pressurechamber 164. The switch 166 consists of an end cap 168 which is formedfrom an electrically conductive material and has an end wall 170 throughwhich a plurality of passages 172 open to permit the fluid medium topass to and fro between the chamber 262 and the chamber 173. The end cap168 also has a tubular wall portion 174 which fits in a close fittingrelationship within the end of the housing 162. An electricallyinsulating collar 176 is located inwardly from the tubular wall 174. Aflexible diaphragm 178 is mounted within the switch 166 and is retainedtherein by means of a retaining wall 180. The retaining wall 180 has aplurality of passage ways 182 which open therethrough such that thefluid pressure within the pressure chamber 164 is applied to one side ofthe diaphragm 178. A transmitter and battery (not shown) may be locatedwithin the pressure chamber 164 with electrical connections being madebetween the battery, the terminals and the electrically conductiveretaining wall 180 and the electrically conductive end cap 168. It willbe seen that the collar 176 which is made from an insulating materialserves to prevent direct electrical contact between the retaining wall180 and the end cap 170. The diaphragm 178 is made from an electricalconductive material and is in contact with the retaining wall 180 whenit is in its normal position shown in FIG. 9. It will be understood thatwhen the pressure in the pressure chamber 164 exceeds the pressure inthe inflation chamber by an amount such that a predetermined pressuredifferential exists between the pressure chamber 164 and the inflationchamber of the pneumatic tire in which the device is mounted in use, thediaphragm 178 will be deflected into contact with the end wall 170 ofthe end cap 168 to complete the electrical circuit and thereby activatethe transmitter as previously described.

An end cap 260 is mounted on the housing 162 and serves to form anantechamber 262 which communicates with the diaphragm 178 through thepassage 172. A small access passage 264 opens into the antechamber 262.The access passage 264 serves to ensure that the pressure in theantechamber 262 will not increase or decrease at a rate which is asgreat as the rate at which the pressure in the inflation chamber of thetire changes in response to movement of the wheel along a rough roadsurface or the like. The addition of the end cap serves to provide a"damper" which prevents the application of "spike" pressures to thediaphragm. It will be noted, however, that the passage 264 issufficiently large to ensure that the pressure in the antechamber 262will increase at a rate substantially in excess of that required inorder to indicate the hazardous rate of change in the inflation chamber.

A second wall assembly of the pressure chamber 164 is generallyidentified by the reference numeral 184. This second wall has a firstprotuberance 186 which projects into the pressure chamber 164. Acharging passage 188 extends through a second wall 184 and openslaterally outwardly through the site of the protuberance 186. A tubularsleeve 190 which is formed from a substantially impermeable elasticmaterial extends over the protuberance 186 and serves to close theopening formed by the through passage 188. A similar protuberance 192projects outwardly from the divider wall 184 and has a discharge passage194 opening therethrough which is normally closed by a sleeve 196. Afilter 198 extends over the inner face of the second wall 184 and servesto filter the fluid medium before it may be discharged from the pressurechamber through the discharge passage 194. A similar filter 200 extendsover the outer face of the second wall 184 and serves to filter thefluid medium before it can pass through the charging passage 188.

A conventional air valve stem 202 has its inner end flange 204 seated inthe chamber 206 so that it bears against the end wall 208. Throughpassage 210 extends through the stem 202 and has a threaded outer endportion 212 which serves to accommodate a Schrader valve externalthreads 214 provided to receive the conventional end cap bearing threads216 are provided to facilitate the mounting of the valve in aconventional manner. A spacer 218 separates the flange 204 of the valvestem from the filter 192 and serves to form a further chamber 220. Itwill be understood that the pressure chamber 164, chamber 206 andchamber 220 each have a circular cross-sectional configuration or anyother desired cross-sectional configuration. In use the pressure sensorsystem 160 is mounted on the rim of a wheel with the valve stem 202projecting outwardly therefrom and the housing 162 located within theinflation chamber of the tire. A conventional one way valve is locatedin the threaded outer end 212 and the tire may then be inflated in aconventional manner. Air will then pass through the through passage 210to enter the chamber 206. The air entering the chamber 206 will bedischarged through the passages 222 into the inflation chamber of thetire. As the pressure within the pressure chamber of the tire increases,air will enter the chamber 220 through the passage 224. This air willpass through the filter 200 to the charging passage 188. Eventually thepressure in the chamber 220 will be sufficient to extend the sleeve 190to an extent sufficient to open the access passage 188 to the pressurechamber 164. Air will continue to pass through the charging passage 188until the pressure differential between the chamber 220 and the pressurechamber 164 is such that the sleeve 190 contracts to close the accesspassage. It will be noted that the sleeve 190 will act to prevent thepressure in the pressure chamber 164 rising above the pressure in thechamber 220 during the initial inflation stage and will serve tomaintain the pressure chamber 164 at a pressure which is slightly lessthan that of the inflation chamber. The pressure differential which ismaintained by the sleeve 190 is greater than that required to cause theflexible diaphragm 178 to be deflected into contact with the retainingwall 180. As a result during the initial inflation stage, the diaphragm178 will be deflected into contact with the retaining wall 180 and willbe supported by the retaining wall 180. The retaining wall 180 serves toinsure that the diaphragm 178 will not be damaged in circumstances wherethe pressure differential between the inflation chamber and the pressurechamber 164 is greater than that which might otherwise be considered tobe a hazardous differential. It will be understood that during theinitial inflation of a pneumatic tire, it is quite common to inflate thetire to a pressure which is substantially above the normal operatingpressure in order to insure that the tire is properly seated on thewheel rim. After the tire has been inflated to the installationpressure, air may then be discharged through the air valve and as aresult the pressure in the tire inflation chamber may drop well belowthe pressure in the pressure chamber 164. When this occurs, the air maythen be vented from the pressure chamber 164 through the dischargepassage 194 by deflecting the sleeve 196. This discharging will continueuntil the pressure in the inflation chamber drops below the normaloperating pressure. As a result the pressure differential between thepressure chamber 164 and the tire inflation chamber is such that thepressure in the chamber 164 exceeds the pressure in the inflationchamber by an amount which greater than that required to deflect thediaphragm 178 into contact with the end wall 170 and these will activatethe transmitter. The tire is then re-inflated to increase the pressurein the inflation chamber to the required operating pressure. As aresult, the pressure in the inflation chamber will once again exceedthat in the pressure chamber. While the pressure differential is reducedby the passage of air through the charging passage 188, the pressure inthe pressure chamber will remain slightly below that of the inflationchamber and consequently, the transmitter will be deactivated.Thereafter the permeability of the diaphragm will permit a gradualelimination of the pressure differential and the diaphragm will returnto the neutral position. As a result of this procedure, the presence ofan operable alarm device is signalled by the fact that the alarm isactivated when air is vented from the overinflated tire. The alarm is,however, deactivated when the tire is reinflated to the operatingpressure and will only be reactivated when a hazardous pressure dropoccurs in the inflation chamber.

If as a result of a fast leak, the pressure in the tire inflationchamber drops at the hazardous rate which exceeds the rate ofpermeability of the diaphragm 178. The diaphragm 178 will be deflectedinto contact with the end wall 172 to activate the transmitter aspreviously described which will in turn activate the alarm system. As aresult, an alarm signal will be generated. If, on the other hand, thepressure in the air inflation chamber of the tire increases or decreasesat a rate which is less than that which is indicative of a hazardousrate of change, the permeability of the diaphragm 178 will insure thatthe diaphragm 178 is not deflected into contact with the end wall 178,and as a result the transmitter will not be activated.

The pressure sensor system illustrated in FIG. 9 can be installed in anypneumatic tire regardless of the cold inflation pressure required in thetire. The cold inflation pressure usually ranges from 28 to 30 psi forautomobiles and 80 to 100 psi for truck tires. The charging passage 188and the discharge passage 194 and their associated sleeves 190 and 196serve to allow the pressure in the pressure chamber to be automaticallyadjusted to match the pressure in the tire and as a result, the sensordevice of the present invention could just as easily be used in a highpressure or low pressure system because the pressure differential acrossthe diaphragm will never exceed that permitted by the sleeves 190 and196 which act as valves that open and close the charging and dischargingpassages 188 and 194.

From the foregoing, it will be apparent that the pressure sensor systemof the present invention is armed and ready to respond to a hazardouspressure change without the need to be electrically live. The systemdoes not draw power from its electrical battery until the transmitter isactivated as a result of the detection of a hazardous rate of pressurechange in the tire. As a result, the operating life of the sensor systemof the present invention may equal that of the tire in which it isinstalled in use. Obviously in circumstances where a defective tire isreplaced or repaired, it is a simple matter to replace the sensor,and/or its battery. A further modification of the present invention asillustrated in FIG. 10 of the drawings wherein the rate of change sensor230 as a pressure chamber B that is separated from the chamber A bymeans of a diaphragm 278. The end wall 232 is made permeable byproviding a small orifice 234 which opens therethrough. In this devicethe diaphragm 278 is made from an electrical conductive material and thelimit plates 280 and 282 are both electrically conductive and formterminals of a switch. When the pressure in the chamber B exceeds thepressure in the chamber A the diaphragm 278 may be deflected intocontact with the wall 280 and when the pressure in the chamber A exceedsthe pressure in the chamber B, the diaphragm may be deflected intocontact with the wall 282.

The container 233 which is diagrammatically illustrated in FIG. 10 maybe used for the purposes of calibrating the sensor switch of the presentinvention. When the capacity of the container 233 and the displacementof the plunger 234 are known, it is possible to change the pressure inthe chamber A at a predetermined rate in order to determine thepermeability of the permeable wall of the chamber B to insure that thediaphragm 278 will be displaced into contact with the wall 280 or thewall 282 when a hazardous rate of change in the pressure differentialbetween the chambers A and B occurs.

A further embodiment of the present invention associated in FIG. 11 ofthe drawings wherein the permeable wall of the pressure chamber B isprovided by forming passages 236 in an impermeable end wall 238 andapply a permeable cover plate 240 to the end wall 238. By reason of thisstructure, it is not necessary to make the diaphragm 278 from apermeable material.

In a further embodiment illustrated in FIG. 12 the housing 230 islocated in a filter housing 242 such that antechambers 244 and 246 areprovided for the chambers A and B respectively. Filter elements 248 and250 serve to filter the fluid medium before it can enter the chambers Aand B respectively.

Yet another embodiment of the present invention is illustrated in FIG.13 of the drawings wherein flexible bellows 252 and 254 enclose theantechamber 244 and 246 respectively. This structure is suitable for usein an environment where it is desirable to exclude the fluid medium inwhich the sensor is operating from the fluid medium in the pressuresensor system. It will be apparent that if the sensor system 260 of FIG.13 is located in a pressure sensitive system in which the pressure dropsthe bellows 252 and 254 will both expand. If the rate at which thepressure drops is a hazardous rate, a pressure differential will beestablished between the chamber A and the chamber B because the passage234 will not permit an equilibrium to be maintained between the chambersA and B; and as a result, the diaphragm 278 will be deflected intocontact with the support valve 280 which will, in turn, activate theelectrical system to generate an alarm. Similarly, if the pressure inthe system in which the sensor 260 is located increases at a hazardousrate, the pressure in the chamber A will increase at a greater rate thanthe pressure in the chamber B; and as a result, the diaphragm 278 willbe deflected into contact with the retainer wall 282 which will againcomplete the electrical circuit to activate an alarm.

In the further modification illustrated in FIG. 14, a temperaturesensitive switch 290 is provided in parallel with the switch 10. Theswitch 290 may be in the form of a normally open bimetallic switch whichis temperature sensitive and will close to complete the circuit andgenerate an alarm when the temperature in the fluid medium in which itis located rises above a predetermined limit. As previously indicated,the sensor 10 compensates for temperature changes in the system andwhile this has distinct advantages, there are many applications in whichit is desirable to generate an alarm when the temperature in the systemexceeds a predetermined limit. For example, when a vehicle is operatingwith dual tires, one of the two tires of a set may be overloaded and mayheat up until it explodes. The system illustrated in FIG. 14 willgenerate an alarm even if the pressure sensor is not activated by thechange in pressure in the system.

As previously indicated, the pressure sensor of the present inventionresponds to the rate of change of pressure in the system which itmonitors and is not affected by the actual pressure in the system. Inaddition, the pressure sensor is not affected by temperature changes inthe system which it monitors because the sensor is housed within thesystem and temperature changes in the fluid which is being monitoredwill result in corresponding temperature changes in the sensor. Thisfeature is particularly desirable when the sensor is used in a pneumatictire where the temperature may vary substantially due to ambienttemperature conditions or speed, load and road surface conditions. Whenthe pressure sensor is mounted within the inflation chamber of a tire,pressure changes due to temperature changes will be substantially thesame in the pressure chamber and in the inflation chamber and will notactivate the alarm. In other systems where the tire pressure is comparedto outside atmospheric pressure the changes in pressure resulting fromtemperature changes could cause the system to generate a false alarm. Inthese systems a drop in ambient temperature could also generate a falsealarm.

A further embodiment of the present invention is illustrated in FIGS.15, 16 and 17 of the drawings to which reference is now made. Withreference to FIG. 15 of the drawings, reference numeral 300 refersgenerally to a pressure sensor constructed in accordance with thefurther embodiment of the present invention. The pressure sensor 300consists of a housing which is generally identified by the referencenumeral 302 and consists of a bottom cover 304 and a top cover 306. Aledge 308 is formed around the open end of the bottom cover 304. Agasket 310 is seated on the ledge 308 and a divider wall 312 rests onthe gasket 310. The top cover 306 has a lip portion 314 arranged to bearagainst the divider wall 312 to clamp the divider wall against thegasket 308.

The divider wall 312 serves to divide the interior space of theassembled housing into a pressure chamber 316 and an input or chargingchamber 318. The top cover 306 has a plurality of air input passages 320opening therethrough and a filter 322 as arranged to overlie thepassages 320 such that any air that enters the input chamber 318 isfiltered.

As shown more clearly in FIGS. 16 and 17, the divider wall 312 isrectangular in shape as is the housing 302. The divider wall 312 has anupper face 324 and a lower face 326. Major passages 328, 330, and 332open through the divider wall 312 as do minor passages 334, 335, 336,and 338.

As shown in FIG. 15 of the drawing, the stem 340 is mounted in thepassage 328 and has a main body portion 342 which projects into theinput chamber 318. A vent chamber 344 is formed in the stem 342 andopens into the pressure chamber 316. An annular recess 346 which isformed on the exterior of the circular stem 342 and serves to form anannular channel 348. Annular shoulders 350 are formed on stem 342 atopposite ends of the channel 348. Vent passages 352 open radially fromthe vent chamber 344 into the channel 348. A tubular sleeve 354 at itsopposite ends supported by the shoulders 350 of the stem and the sleeveextends over the channel 348 and serves to close off the channel 348.The tubular sleeve 354 is made from a resiliently flexible electricallyconductive material. The tubular sleeve 354 is proportioned so that theopposite ends thereof must be stretched in order to fit over theshoulders 350 with the result that in the relaxed configuration the mainbody of the sleeve will assume the concave curvature illustrates in FIG.15. It iwll be noted, however, that if the pressure in the channel 348is increased, the sleeve may be caused to balloon outwardly andultimately the pressure in the channel 348 may be relieved by separationof the sleeve from one or both of the shoulders 350 to permit the fluidmedium to be discharged from the channel 348 into the chamber 318.

The stem 340 and tubular sleeve 354 are both electrical conductors. Thesleeve 354 forms one terminal of an electrical switch. The otherterminal of the electrical switch is formed by a conductive bridgemember 360 which has legs 362 and 364 which extend through the passages335 and 334 (FIG. 17) of the divider wall. The main body portion 366 ofthe conductive bridge is in the form of a cylindrical member which hasan annular flange 368 which protects radially inwardly and has a passage370 opening therethrough. The inner face 372 of the passager 370 formsthe second terminal of a switch. The switch is closed when theconductive sleeve 354 is inflated to the extent that it will expandoutwardly into contact with the face 342 of the conductive bridge. Asshown in FIG. 18 of the drawings, the printed circuit has paths 374 and376 that communicate with the conductive bridge 360 and the stem 340respectively.

The conductive sleeve 354 is made from a permeable material of the typepreviously described with reference to the permeable diaphragm of theearlier embodiments such that the fluid medium that is contained in thepressure chamber 316 may pass through the tubular sleeve at a rate whichwill serve to permit equilization of the pressure on opposite sides ofthe diaphragm at a rate that would prevent detection of a change inpressure in the pressure sensitive system which occurs at a rate that isless than a hazardous rate. The permeable sleeve is sufficientlyimpermeable to prevent the passage of the fluid medium therethrough at arate which will serve to prevent equalization of the pressure onopposite sides of the sleeve at a rate which would prevent movement ofthe sleeve from the retracted postition shown in FIG. 15 to the extendedposition when a hazardous rate of change occurs in the pressuresensitive system. From the foregoing it will be seen that the tubularsleeve of this embodiment performs the function of the flexiblediaphragm and the first pressure relief valve of the embodimentillustrated in FIG. 9. In addition, it will be seen that the P.C. board36 performs the function of the divider wall.

To supplement the permeability of the sleeve 354, a permeable plug 380is mounted in the passage 332. This permeable plug 380 has the samepermeability characteristics as those previously described withreference to the sleeve 354 and may be used either to supplement thepermeability of the sleeve or in circumstances where the sleeve 354 ismade from an impermeable material.

An input stem 390 is mounted in the passage 330 and has an input chamber392 which opens into the input chamber 318. Passages 394 open radiallyoutwardly from the chamber 392. A tubular sleeve 396 is mounted on thestem 390 and serves to normally close the passages 394 so that it willfunction as a charging device similar to that previously described withreference to FIG. 9.

As shown in FIG. 16 of the drawings, a chip 398 and a transmitter 400are mounted on the circuit board and are connected to the printedcircuit through the passages 336 and 338 respectively.

In use when a hazardous reduction in pressure occurs in the chamber 318,the pressure differential across the sleeve 354 will cause the sleeve354 to balloon outwardly to contact the conductive bridge and this willcomplete the circuit of the printed circuit board which will in turnactivate the transmitter 400 which will transmit a signal to the alarmdevice which will activate the alarm.

FIG. 18 of the drawings illustrates a modified stem and tubular sleeveconstruction. In the embodiment illustrated in FIG. 18, two tubularsleeves are employed. The inner tubular sleeve 500 is a highly flexiblenon-conductive sleeve and the outer sleeve 502 is a conductive sleeve.The proximal end 504 of the inner sleeve is seated in a groove 506 whichis formed in the stem 508. The proximal end of the outer sleeve 502rests on the shoulder 510 of the stem and makes electrical contact withthe stem through the shoulder 510. By using this two part sleevestructure it is possible to use a relatively thin walled conductivesleeve which will have a substantial degree of flexibility whilemaintaining its electrical conductivity. By using a highly flexiblenon-conductive sleeve 500 it is possible to ensure that the passage 512can be closed efficiently in use. This structure can be used in theembodiment illustrated in FIG. 15 when a permeable plug 380 is used.

An alternative structure is illustrated in FIGS. 19 and 20 of thedrawings. As shown in FIG. 19, the stems 520 may be of identicalconstruction and may be made from an extruded plastic material. Thestems 520 do not have to be electrically conductive and similarly theflexible sleeves 522 are not made from electrically conductive material.As a result, the stem structure is inexpensive to manufacture and simpleto install.

The diaphragm 524 is made from a conductive material and is secured tothe P.C. Board by a conductive casing 526. A conductive bridge 528 ismounted on the underside of the P.C. Board 312 and extends in a spacedrelationship with respect to the wall 530 of the casing 526. Theconductive diaphragm 526 is sufficiently flexible to be deflected to theposition shown in FIG. 19 when a predetermined pressure differential isestablished in order to complete the circuit of the P.C. Board and toactivate an alarm as previously described.

A still further embodiment of the present invention is illustrated inFIG. 21 of the drawings. In this embodiment, a pressure transducer 600is mounted in the passage 602 which opens through the P.C. Board 604.The pressure transducer is in the form of a transducer crystal which ismade permeable by forming one or more passages that open therethrough.The passages may be formed by a laser so as to provide the degree ofpermeability previously described in the other embodiments of theinvention which will ensure that the pressure transducer crystalresponds to the rate of change of pressure in the pressure sensitivesystem in a like manner to that previously described. Alternativley, theP.C. Board may be made permeable by forming one or more passages openingtherethrough by means of a laser in order to provide the requiredpermeability.

Various further modifications of the present invention over and abovethose described herein will be apparent to those skilled in the art.

I claim:
 1. A pressure sensor switch comprising:(a) a stem having a sidewall surrounding a vent chamber that opens into one end of the stem, avent passage opening from the vent chamber through the side wall of thestem, (b) a tubular sleeve mounted on said stem and overlying said ventpassage, said tubular sleeve being flexible so as to be movable to andfro between a first position in which it extends in close proximity tosaid vent passage and a second position which is spaced outwardly fromthe first position with respect to the step in response to a variationin pressure in the vent chamber, said sleeve being electricallyconductive and forming a first terminal of said switch, (c) a secondterminal of said switch bieng located opposite said tubular sleeve so asto be spaced therefrom when the sleeve is in the said first position andto contact the sleeve when the sleeve is in said second position to openand close said switch in response to movement of said sleeve.
 2. Apressure sensitive switch as claimed in claim 1 wherein the stem has anouter face in which an annular recess is formed between first and secondananular shoulders, said vent passage opening from said vent chamberinto said annular recess, said tubular sleeve bridging and closing theannular recess and having opposite ends thereof mounted on said annularshoulders.
 3. A pressure sensitive switch as claimed in claim 2 whereinsaid second terminal has a contact surface surrounding said sleeve.
 4. Apressure sensitive switch as claimed in claim 3 wherein a portion of thesecond contact surface of the second terminal is located opposite saidvent passage.
 5. A pressure sensitive switch as claimed in claim 2wherein at least one of said opposite ends of said sleeve is resilientlydeformable to separate from the shoulder on which it is mounted topermit the fluid medium to discharge from the annular recess when thepressure in the pressure chamber exceeds a predetermined valve.
 6. Apressure sensitive switch as claimed in claim 1 further comprising ahousing having a divider wall therein that separates a pressure chamberfrom an input chamber, said stem being mounted on said divider wall suchthat said vent chamber communicates with said pressure chamber and saidtubular sleeve is located in said input chamber and separates the ventpassage from the input chamber.
 7. A pressure sensitive switch asclaimed in claim 6 wherein said divider wall is a printed circuit boardwhich has a circuit printed thereon which is electrically connected tothe first and second terminals.
 8. A pressure sensitve switch as claimedin claim 7 wherein the stem is an electrical conductor that communicatesbetween the circuit of the printed circuit board and the tubular sleeve.9. A pressure sensitive switch as claimed in claim 1 wherein saidtubular sleeve is fluid permeable member which is sufficiently permeableto permit the fluid medium to pass therethrough at a rate that willserve to permit equalization of the pressure in the pressure chamber andin the input chamber to prevent displacement of the tubular member fromthe first position to the second positon when the rate of change in thepressure and the input chamber is less than that which is indicative ofa predetermined hazardous rate of change, the permeable member beingsufficient impermeable to prevent the passage of fluid mediumtherethrough at a rate that will serve to prevent equalization of thepressure in the pressure chamber and the input chamber at a rate thatwould prevent movement of the sleeve from said first position to saidsecond position when the predetermined rate of change occurs in thechamber.
 10. A pressure sensitve switch as claimed in claim 6 wherein apressure balancing passage is formed in the divider wall thatcommunicates between the pressure chamber and the input chamber, and afluid permeable member is mounted in said pressure balancing passage,said fluid permeable member being sufficiently permeable to permit thefluid medium to pass therethrough at a rate that will serve to permitequalization of the pressure in the pressure chamber and in the inputchamber to prevent displacement of the tubular member from the firstposition to the second position when the rate of change in the pressureand the input chamnber is less than that which is indicative of apredetermined hazardous rate of change, the permeable member beingsufficient impermeable to prevent the passage of fluid mediumtherethrough at a rate that will serve to prevent equalization of thepressure in the pressure chamber and the input chamber at a rate thatwould prevent movement of the sleeve from said first position to saidsecond position when the predetermined rate of change occurs in thechamber.
 11. A pressure sensitive switch as claimed in claim 10 whereinthe fluid permeable member is in the form of a plug which is seated insaid pressure balancing passage.
 12. A pressure sensitive switch asclaimed in claim 6 wherein the said housing comprises first and secondsegments which cooperate with one another to secure the divider walltherebetween.
 13. A pressure sensitive switch as claimed in claim 6wherein a second stem is mounted on said divider wall, said second stemhaving a side wall in which an input passage is formed which has a firstportion of its length that opens inwardly from the input chamber and thesecond portion of its length which opens through the side wall into thepressure chamber and a second tubular sleeve mounted on said second stemand overlying and normally closing said second portion, said secondtubular sleeve being flexible so as to be movable with respect to thesecond stem to permit the fluid medium to pass therethrough when thepressure in the input chamber is substantially greater than apredetermined operating pressure.